1. Field of the Invention
The present invention concerns the field of material and compositions to shield and contain radioactive substances and radioactive substances in particular.
2. Background of the Invention
For some years, especially following the near "melt down" of the Chernobyl Power Station reactor, there has been considerable international antipathy or downright hostility towards nuclear energy. This is despite the demonstrated and growing danger of global climate change resulting from the atmospheric effects of burning fossil fuels. The primary opposition to nuclear energy stems from the seemingly insurmountable hazards and environmental damage resulting from the long-lived radioactive wastes produced by current nuclear reactors. Yet the potential environmental damage of nuclear wastes must be some how balanced against the certain environmental damage of continued use of fossil fuels.
It appears clear that the only way to avoid the environmental catastrophe posed by global warming--short of returning to a preindustrial economy--is to replace conventional power sources with ones based on nuclear fission. At some future date "dirty" fission-based power sources may be replaced with cleaner fusion-based systems, but at this time nuclear fission seems to only option. Because we do not currently know of any way to eliminate nuclear waste, our goal must be the safe handling and containment of this waste. The current nuclear fuel cycle presents a number of operations that are potentially environmentally adverse. These include the mining and manufacture of nuclear fuels, the fission of these fuels and the hazards presented by operating reactors, the on site storage of spent fuel, the transport and recycling or disposal of these fuels.
It appears that safe reactors are within the grasp of human engineering. The real environmental problem is posed by the recycling and disposal of the spent nuclear fuels. Whether the spent fuels are reprocessed to yield additional fissionable material (the most efficient alternative from the view of long term energy needs) or whether the spent fuel is simply disposed of directly, there is a considerable volume of highly radioactive substances that must be isolated from the environment. The presently acceptable approach is the internment of the radioactive material in deep geologic formations where they can decay to a harmless level without any human intervention. Ideally these "buried" wastes must remain environmentally isolated with no monitoring or human supervision. Otherwise any disruption of human civilization might lead to a catastrophic escape of radioactive materials. That is, one does not simply dump the wastes in a hole. These materials are constantly generating heat; further potentially explosive gases, primarily hydrogen, are also generated. The emitted radiation alters and weakens most materials. Presently the best approach is to reduce the wastes to eliminate solvents. The reduced wastes are then vitrified or otherwise converted into a stable form to prevent environmental migration. Nevertheless, there remains the important task of producing special materials that display unusual resistance to radiation, heat and chemical conditions that generally accompany radioactive wastes. Ideally, such materials have radiation shielding properties and can be used to shield and incase otherwise reduced wastes. Another important application of such materials is the sealing of decommissioned or damaged nuclear facilities.
The simplest and crudest of such materials is probably concrete. Because of the mineral inclusions in simple portland cement based materials or similar materials to which additional shielding materials (e.g. heavy metal particles) these substances provide shielding of nuclear radiation. However, simple concrete may not long survive under the severe chemical conditions provided by some nuclear wastes. Concrete tanks of liquid nuclear wastes have useful lifetimes of less than fifty years. Concrete is more effective against reduced vitrified wastes but is still far from ideal. There have also been a number of experiments with novel shielding-containment materials that would be easier to apply and have superior shielding and/or physical properties. However, until now these materials have not proven widely successful.